The present invention relates generally to a light generator, and more particularly to a light source used to transfer a fine pattern in the semiconductor manufacture. The present invention is suitable, for example, for an exposure apparatus that utilizes the X-ray and the extreme ultraviolet (“EUV”) light as a light source.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern of a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of the light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with the recent demands for the finer semiconductor devices, a shorter wavelength of ultraviolet (“UV”) light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the UV light has to satisfy the rapid advancement of the fine processing of a semiconductor device, and a reduction projection optical system using the EUV light with a wavelength of 10 to 15 nm shorter than that of the UV light (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less. The EUV light source uses, for example, a laser plasma light source that irradiates a high-intensity pulsed laser light onto a target material, such as a metal thin coating, inert gases and liquid drops, in the vacuum chamber, generates the high-temperature plasma, and uses the EUV light having a wavelength of, for example, about 13 nm (see, for example, Japanese Patent Application, Publication No. 2002-174700, and Richard H. Moyer et al., Proceeding of SPIE Vol. 4343 (2001), pp. 249–251.
The laser plasma as one mode of the EUV light source irradiates the high-strength pulse laser light onto the target material and generates not only the EUV light from the target material but also flying particles called debris, which are made of gasified, liquidized and ionized target materials, and cause contaminations, damages and lowered reflectance of an optical element. A supply mechanism that supplies the target material also emits the debris (flying particle).
When the debris attaches to the laser optical system that introduces the pulsed laser light to the target material, the attached debris scatters and absorbs the pulsed laser beam. This configuration cannot supply the energy necessary to generate the plasma to the target material and stably generate the EUV light.